But proper, tailored pants with proper, fitted waistbands, made in wool or crepe or silk satin or lace. Some are jeweled, many brocade, a few rubberized, and just about all of them will end in tragedy should you inadvertently toss them in the wash.

Pants, make no mistake, are the big story for fall: from classic, front-pleated, masculine strides at Ralph Lauren and Armani to side-zippered, slit-at-the-ankles capris at Chloé; from leathery and/or jodhpury at Givenchy, Hermès, and Gucci to printed and luxuriously ethnic at Oscar de la Renta (think Babe Paley and her gal pals staying at some raj's palace in Jaipur). And let's not forget the geometric-patterned and jeweled and kick-flared pantsuits at Prada and Louis Vuitton. As if we could. Miu Miu's show was almost entirely a pantsuit fest. Sometimes the pants and blazers matched. Sometimes they didn't. Sometimes the blazers were coats.

That's how it is with pantsuits these days.

Should we be worried? Thanks to Marlene Dietrich, Kate Hepburn, and Coco Chanel, pants-wearing has been the cornerstone of gamine sexiness since about 1930. As markers of social and economic change in women's status, they're unparalleled. However, let's be honest, the question "Will my thighs look fat?" clearly takes precedence over everything. But we're getting ahead of ourselves.

While I was musing over this piece, I kept coming across women in pants, whether it was Hillary at her most Clintonesque; Michelle Obama and Samantha Cameron, the wife of the British prime minister, touring around Washington, D.C.; or France's de facto first lady, Valérie Trierweiler, drawing a line between herself and her predecessor, the Dior dress—wearing Carla Bruni-Sarkozy. These female pants-wearers weren't off duty. They were networking, dining, and skittering off to board meetings on an international scale—in pants. Miuccia Prada (who never wears pants) took her bow at Miu Miu in an astrakhan pair, and later wore a lemon brocade tunic and matching pants to the Met's Costume Institute Gala.

Pants. Anytime. Anyplace. Really? Yes, really. Like Miuccia Prada, designers are especially excited this season about the evening pant—in velvet brocade and worn with a matching tunic or a peplumed top. The floaty-pajama style that Stella McCartney pioneered a year ago, when she turned up at red-carpet events in printed pants and made everyone else look overdressed at worst, and heavily sedated and ill at ease at best. It's everywhere. Ditto tux pants. "Trousers are pretty much my uniform. I can go to the office or on the school run and feel comfortable in both environments," McCartney tells me.

Since she was clad in sweatpants at the time, I think we can accept her take. She was also wearing heels and one of her own bespoke Savile Row—esque, double-breasted Prince of Wales check jackets. It looked contemporary, smart, and very cool.

Maybe you need to work in fashion to do the sweatpants. But you can still use pants to inject a sophisticated ease into your wardrobe. "If I want to wear a jacket and look elegant, a pair of pants adds the right note of modernity," says Judith Milgrom, the chic, classic designer of the chic, classic French brand Maje. "Trousers mean I can even wear a suit and not look corporate."

Milgrom's sister, Evelyne Chétrite, a designer and cofounder of the equally successful, equally chic brand Sandro, agrees—about both the uniform and the function—of pants. "I never try to look completely 'chic,' " she says. "For me, it's much younger to play with contrasts. I like to wear feminine blouses and jackets with masculine pants."

Nothing lasts forever: The dress has been a great ally these past few years, but there's a collective desire among women to find a garment that telegraphs how attuned we are to the current puritan ethos. Or maybe we just want an excuse to go out and buy a new wardrobe.

Did somebody mention leather pants? I found myself writing about leather pants so often during my fall runway reports that I had to resort to the cunning cryptic code of "LP." Then I had to resort to buying a pair, despite my conviction (since comprehensively eviscerated) that they were for teenage Goths. Once upon a time they were. But the new $1,000 cotton-backed nonbaggy leather pants are superflattering, ultraluxurious, and add a polished edge to classic favorites like trench coats, jackets, and cashmere knits.

"I absolutely don't have a problem wearing them even to quite formal meetings," says Alison Loehnis, the new managing director of Net-a-Porter, of her tailored leather pants from the Row. "If I wanted to make a point about being conventionally smart, I might add a blazer. Then again I might not. A feminine blouse could be enough, especially in a fashion environment." Is there a cutoff age for leather pants? She reflected. She called in a colleague. The colleague reflected. All three of us ruminated good and long. Eventually a unanimous verdict was reached: There is no cutoff. Provided you always wear the right shoe.

The shoe issue: If you want to see shoe poetry in motion, look no further than Phillip Lim's fall catwalk, where cropped-just-above-the-ankle pants were teamed with dainty pointed toes. It's probably no coincidence that we're also seeing a massive return to the high-heeled pump. "It's the perfect shoe with slim trousers because it shows just the right amount of flesh and it's very elegant, so you can play a modern and classic game very successfully," Lucy Choi says happily—happy because as chance would have it, she recently launched an online boutique full of reasonably priced pumps in myriad colors and skins and 10 heel heights, from two to five inches (lucychoilondon.com). "Basically, the longer the pant, the higher the heel."

We need to talk about fit. And fat. Because they go together. Length is pretty important too. Some people say that there are no rules in fashion anymore. They're wrong.

Here are some nonnegotiables: 1) Don't expect to find the perfect fit. That's what in-store alteration services are for. However, while seams can be taken in or let out and hems raised, if the pants don't fit properly around your bottom, quit while you're ahead. 2) Avoid elastic waistbands and drawstring pants no matter how much you like the color or print. They create gathers around your buttocks. That's gross. 3) White pants and prints can be worn by nonskinny women as long as they're not tight. 4) A flare can be very flattering on taller women, balancing out a full thigh. 5) Shorter women should avoid flares and opt instead for a small kick flare at the hem, as seen for fall at Prada and Louis Vuitton. 6) Stretch is never a bad idea. 7) Shine makes legs look bigger. 8) Use blazers and tunics to conceal hips or thighs if necessary. 9) Length is personal, but the longer you go, the more you'll need to consider your heels. Either make sure you always wear the same height heel or buy identical pairs of pants in two lengths. Alternatively, listen to Simon Kneen, the creative director at Banana Republic; he and his team have spent months working out the easiest length for most women. "Just above the ankle—you can wear it with any heel," he explains. "And the ankle is so sexy." 10) Don't be deterred by the rules. Once you've found the perfect cut, buy, repeat. You've done the hard part.